Heater-cathode insulation leakage test method and apparatus

ABSTRACT

A high voltage, short duration pulse is applied between the heater and cathode of an electron tube to cause an electrical breakdown of dielectric material disposed between the heater and cathode when the dielectric material is defective. After the high voltage pulse, a second voltage having a value less than the peak value of the high voltage pulse is applied between the heater and cathode to provide a sustaining current through the dielectric material when, and only when, a dielectric breakdown has occurred. Current measuring means are disposed between the lower voltage source and either the heater or cathode of the tube to measure the leakage current, if any, that is present during the application of the lower sustaining voltage.

United States Patent West Apr. 2, 1974 HEATER-CATHODE INSULATION Primary Examiner-Alfred E. Smith LEAKAGE TEST METHOD AND Assistant ExaminerRolf Hille R TUS Attorney, Agent, or Firm-Edward J. Norton; William [75] Inventor: Clarence Austin West, Somerville, Sqmre NJ. [57] ABSTRACT [73] Asslgnee: RCA m New York A high voltage, short duration pulse is applied be- [22] Fil d; D 11, 1972 tween the heater and cathode of an electron tube to 21 cause an electrical breakdown of dielectric material 1 Appl' 3l4003 disposed between the heater and cathode when the dielectric material is defective. After the high voltage [52] U.S. Cl. 324/20, 324/54 p a Second voltage ha ing a alue l ss than the [51] Int. Cl. G0lr 31/22 P ue o the high voltage pulse is applied be- [58] Field of Search 324/20, 24, 54 tween the heater and cathode to Provide a Sustaining current through the dielectric material when, and only [56] References Cit d when, a dielectric breakdown has occurred. Current UNITED STATES PATENTS measuring means are disposed between the lower voltage source and either the heater or cathode of the 3,488,580 1/1970 Anderson et al. 324/52 2,760,151 4/1956 Andrews et al. 324 20 CR tube to measure the: leakage current that present during the application of the lower sustaining voltage.

5 Claims, 2 Drawing Figures HEATER-CATHODE INSULATION LEAKAGE TEST METHOD AND APPARATUS BACKGROUND OF THE INVENTION The present invention relates to apparatus and a method for detecting electrical insulation current leakage between an indirectly heated cathode and heater electrodes in an electron tube.

In indirectly heated cathode type electron tubes, particularly in conventional television picture tubes, the electron emitting cathode element is electrically insulated from the heater element which indirectly heats the cathode element by conduction through an insulating dielectric material disposed between the heater and cathode electrodes. It is known that chemical impurities in the ceramic or other material forming the dielectric, which serves as the heat conducting and electrical insulating medium between the cathode and heater electrodes, may cause lowering of the electrical resistance of that dielectric material.

In a television picture tube, the lowering of this resistance causes deleterious effects on the operating char.- acteristics of the tube and, therefore, tubes exhibiting these impurities and accompanying lowered resistances should be screened from production assemblies. To so screen the assemblies, the dielectric material disposed between the cathode and heaterelectrodes are normally tested for insulation resistance by impressing the rated operating voltage of the tube across the two electrodes. Should a leakage current be detected in the dielectric material, the tube is rejected. However, it has been found that certain picture tubes have exhibited intermittent lowering of theelectrical resistance of this dielectric material and when subjected to the normal dielectric insulation test, the insulation tests as acceptable. This problem is particularly troublesome when a customers television tubeis said to be intermittent and, when returned. to the factory, the factory tests have indicated a satisfactory tube. To impress a sustaining voltage higher than the factory rated voltage at which the leakage test is normally conducted would cause the destruction of the tube and is not therefore incorporated in presentday'tube testing equipment.

SUMMARY OF THE INVENTION In accordance with the present invention a method is provided for testing an electron tube for insulation current leakage between a first electrode and a second electrode space from the first electrode by a dielectric material disposed therebetween. The method comprises applying a voltage pulse occurring in a time interval having a given duration between the electrodes. The voltage pulse has a peakvalue resulting in a first current flowing throughthe dielectric in the-time interval only when thedielectric is defective. A voltage is applied across the electrodes contiguous with andsubsequent tothe voltage pulse, the voltage having a value less than the peak value. The voltagevalue results in a second current flowing through the dielectric between the electrodes, only-.when the first currentflows. Means for monitoring thesecond current flowing through the dielectric material are provided.

An apparatus for testing an electron tube for insulation leakage between first and second spaced electrodes including a dielectric material disposed between the electrodes includes a pair of output terminals each capable of being connected to a different one of the electrodes. Voltage generating means are coupled to the terminals for impressing a first voltage across the dielectric material between the electrodes in a time interval having a given duration when the terminals are connected to the electrodes. The first voltage has a given peak value resulting in a first current flowing through the dielectric material in that time interval only when the dielectric material is defective. Voltage reducing means are coupled to the terminals for reducing the level of the first voltage to produce a second voltage having a value less than the given peak value. The second voltage value tends to cause a second current to flow through the dielectric material only when the first current flows. Current monitoring means are coupled between the voltage reducing means and one of the electrodes for monitoring the second current flowing through the dielectric material betwen the electrodes.

IN THE DRAWINGS FIG. 1 is a schematic diagram of a circuit constructed and operated in accordance with an embodiment of the present invention. 7

FIG. 2 is a voltage-time curve useful in explaining the operation of the circuit of FIG. 1.

DETAILED DESCRIPTION In FIG. 1, tube 10 under test includes a heater electrode 12 and a cathode electrode 14 between which is disposed a solid dielectric material 16 shown in phantom which electrically insulates electrodes 12 and 14 from each other but which conducts heat from heater 12 to cathode 14. Cathode 14 is serially connected through a current meter 18, voltage dropping resistor 20 and voltage source 22 to heater 12.

Coupled across resistor 20 are a pair of terminals 24 and 26. A second voltage source 28 has a pair of output terminals 30 and 32. Capacitor 34 and resistance 36 are serially connected between terminals 38 and 40. Double pole, double throw switch 42 simultaneously connects terminal 38 with terminal 24 and terminal 40 with terminal 26 or simultaneously connects terminal 38 with terminal 30 and terminal 40 with terminal 32.

With terminal 38 connected to terminal 30 and terminal 40 connected to terminal 32, voltage source 28 charges capacitor 34' to the peak voltage level. With switch 42 in the other switch position such that terminal 38 is connected to terminal'24 and terminal 40 is connected to terminal 26, capacitor 34 discharges into the networkincluding tube 10, current meter 18, resistor 20 and voltage source 22, superimposing upon the voltage supplied by voltage source 22 a voltage pulse.

' The action just described is best seen by examination of curve 'of FIG. 2. In FIG. 2, curve 50 comprises a steady state voltage 52 said a voltage pulse 54. Pulse 54 occurs within time period T. Steady state voltage 52 commences within time period T and continues thereafter. Pulse, 54 is generated by the discharge of capacitor 34, while steady state voltage. 52 is provided by voltage source 22; Voltage 56 shown in phantom is the steady state voltage occurring within the time period T such that pulse 54 when. generated is superimposed thereon as indicated above. Therefore, the voltage peak 58 represents the sum of the voltages of pulse 54 and steady state voltage 52.

A typical television picture tube has a peak heatercathode voltage rating with the heater negative with respect to the cathode of 450 volts maximum during the television tube warm-up period not exceeding seconds. After the tube is warmed, the maximum voltage for the tube is 200 volts. These voltages represent factory rating maximums for typical television picture tubes. During normal factory testing of such tubes for insulation leakage between the cathode and heater electrod es, the maximum voltage applied is no greater than 200 volts in accordance withth rating of that tube. However, where tubes are intermittent with respect to heater to cathode current leakage, such steady state voltages are not always satisfactory for insulation leakage testing.

As provided by an apparatus built and operated in accordance with the present invention, a high voltage is initially applied across the heater and cathode electrodes which voltage is within the rating of the tube specified for the warm up period. As an example, source 22 is 275 volts DC and source 28 is 150 volts DC providing a total peak volts 58 (FIG. 2) of 425 volts. Peak voltage 58 is less than the typical 450 volts warm up rating of the television picture tube. Pulse 54 is applied for period T which is preferably less than the fifteen second warm up period rating of the tube. Period T has a time constant determined by resistor and capacitor 34. Thus no destruction of the tube will occur as a result of the application of peak voltage 58. As shown in MG. 2, pulse 54 decays to the sustaining voltage level within the period T. Resistor 36 is a current limiting resistor of a few ohms to protect circuit elements during the charging of capacitor 34.

When dielectric material 16 in tube 10 is defective and exhibits chemical impurities or other characteristics which cause intermittent operation of the television tube, then the amplitude of pulse portion 54 is sufficient to result in an electrical discharge are through the defective portion of insulating dielectric 16. This arcing takes place across a low impedance path in the dielectric material.

It is believed that once the arcing commences, heat generated thereby causes a lowering of resistance of the arced path. This lowering of the resistance results in the arc sustaining itself. However, since the impedance caused by the arcing has actually been reduced in the dielectric material, then, as provided in accordance with the present invention, the high voltage level of pulse 54 need not be continued. A lower level voltage is then applied between the heater 12 and cathode 14 and will sustain the leakage current through dielectric 16 by way of the lowered resistance path caused by the higher voltage arcing.

By inserting current meter 18 within the low voltage source 22 circuit, any leakage that occurs in tube 10 during the time period of voltage 52 of curve 50 is measured and monitored by meter 18. Resistor 20 provides discharge of capacitor 34 through the heater-cathode circuit of tube 10 and also serves as a current limiter device to protect meter 18 in case of a short circuit through tube 10. In a circuit built and operated in accordance with the present invention for testing a television picture tube, some typical values for the various circuit elements include a voltage source 22 of 275 volts DC, a voltage source of 136 vol ts DC: a resistance 36 of 10 ohms, a capacitance 34 of 16 microfarads and a resistance 20 of 50 K ohms. Perod T is less than fifteen seconds. Depending on the electron tube ratings, the voltage sources 22 and 28 can be adjust- 5 able. Likewise, the value of resistance 20 or of impedance 34 can be varied to provide the proper period T.

What is claimed is:

l. A method for testing an electron tube for insulation current leakage between a first electrode and a second electrode spaced from said first electrode by a dielectric material disposed therebetween, comprising:

reducing the level of said first voltage to produce a second voltage having a value less than said given peak value, said second voltage value tending to cause a second current to flow through said dielectric material only when said first current flows, and

monitoring the magnitude of said second current flowing through said dielectric material between said electrodes produced by said second voltage.

2. The method of claim 1 wherein said impressing step includes the step of superimposing a voltage pulse on said second voltage.

3. A method for testing an electron tube for insulation current leakage between a first electrode and a second electrode spaced from said first electrode by a dielectric material disposed therebetween comprising:

impressing a voltage pulse occurring in a time interval having a given duration across said electrodes,

said voltage pulse having a peak value resulting in a first current flowing through said dielectric in said time interval only when said dielectric is defective,

impressing a voltage across said electrodes contiguous with and subsequent to said voltage pulse, said voltages having a value less than said peak value, said voltage value resulting in a second current flowing through said dielectric between said electrodes, only when said first current flows, and

monitoring said second current flowing through said dielectric material.

4. An apparatus for testing an electron tube for insulation leakage between first and second spaced electrodes including a dielectric material disposed between said electrodes, comprising:

a pair of output terminals each being capable of being connected to a different one of said electrodes,

voltage generating means coupled to said terminals for impressing a first voltage across said dielectric material between said electrodes in a time interval having a given duration when said terminals are connected to said electrodes, said first voltage having a given peak value resulting in a first current flowing through said dielectric material in said time interval only when said dielectric material is defective,

voltage reducing means coupled to said terminals for reducing the level of said first voltage to produce a second voltage having a value less than said given peak value, said second voltage value tending to cause a second current to flow through said dielectric material only when said first current flows, and

a pair of output terminals each being capable of being coupled to a different one of said electrodes,

pulse generating means coupled to said terminals for impressing across said dielectric material when said terminals are connected to said electrodes a voltage pulse occurring in a time interval having a given duration, said voltage pulse having a peak value resulting in a first current flowing through said dielectric in said time interval only when said dielectric is defective,

voltage generating means coupled to said terminals for impressing a voltage across said dielectric material contiguous with and subsequent to said voltage pulse when said terminals are connected to said electrodes, said voltage having a value less than said peak value, said voltage value resulting in a second current flowing through said dielectric between said electrodes only when said first current flows, and

leakage current monitoring means coupled between one of said terminals and said voltage generating means for monitoring said second current. 

1. A method for testing an electron tube for insulation current leakage between a first electrode and a second electrode spaced from said first electrode by a dielectric material disposed therebetween, comprising: impressing a first voltage across said dielectric material between said electrodes in a time interval having a given duration, said first voltage having a given peak value resulting in a first currEnt flowing through said dielectric material in said time interval only when said dielectric material is defective, reducing the level of said first voltage to produce a second voltage having a value less than said given peak value, said second voltage value tending to cause a second current to flow through said dielectric material only when said first current flows, and monitoring the magnitude of said second current flowing through said dielectric material between said electrodes produced by said second voltage.
 2. The method of claim 1 wherein said impressing step includes the step of superimposing a voltage pulse on said second voltage.
 3. A method for testing an electron tube for insulation current leakage between a first electrode and a second electrode spaced from said first electrode by a dielectric material disposed therebetween comprising: impressing a voltage pulse occurring in a time interval having a given duration across said electrodes, said voltage pulse having a peak value resulting in a first current flowing through said dielectric in said time interval only when said dielectric is defective, impressing a voltage across said electrodes contiguous with and subsequent to said voltage pulse, said voltages having a value less than said peak value, said voltage value resulting in a second current flowing through said dielectric between said electrodes, only when said first current flows, and monitoring said second current flowing through said dielectric material.
 4. An apparatus for testing an electron tube for insulation leakage between first and second spaced electrodes including a dielectric material disposed between said electrodes, comprising: a pair of output terminals each being capable of being connected to a different one of said electrodes, voltage generating means coupled to said terminals for impressing a first voltage across said dielectric material between said electrodes in a time interval having a given duration when said terminals are connected to said electrodes, said first voltage having a given peak value resulting in a first current flowing through said dielectric material in said time interval only when said dielectric material is defective, voltage reducing means coupled to said terminals for reducing the level of said first voltage to produce a second voltage having a value less than said given peak value, said second voltage value tending to cause a second current to flow through said dielectric material only when said first current flows, and current monitoring means coupled between said voltage reducing means and one of said electrodes for monitoring said second current flowing through said dielectric material between said electrodes.
 5. An apparatus for testing an electron tube for insulation current leakage between a first electrode and a second electrode spaced from said first electrode by a dielectric material disposed therebeween, comprising: a pair of output terminals each being capable of being coupled to a different one of said electrodes, pulse generating means coupled to said terminals for impressing across said dielectric material when said terminals are connected to said electrodes a voltage pulse occurring in a time interval having a given duration, said voltage pulse having a peak value resulting in a first current flowing through said dielectric in said time interval only when said dielectric is defective, voltage generating means coupled to said terminals for impressing a voltage across said dielectric material contiguous with and subsequent to said voltage pulse when said terminals are connected to said electrodes, said voltage having a value less than said peak value, said voltage value resulting in a second current flowing through said dielectric between said electrodes only when said first current flows, and leakage current monitoring means coupled between one of said terminals and said voltage generating means for monitoring said second cuRrent. 